In recent years, image pick-up elements such as CCD devices have achieved high resolution and become compact as a result of advances in fine pattern technology which have enabled the pick-up element pixel size to be reduced. As a result of these advances, highly efficient and much smaller-sized objective optical systems have been in demand in optical systems which employ such image pick-up elements as, for example, endoscope systems. Especially in the case where an optical system employs, for example, a beam-folding prism or mirror which serves as a visual beam conversion element to provide a side or perspective view, there is the tendency for the entire optical system to become large as compared with an optical system that provides only a direct view. Thus, a size-reduction in such an optical system is desired. Moreover, while small size and high performance of an optical system are desired, nevertheless, an inexpensive objective optical system with three or fewer lens elements is also desired.
As a prior art example of an objective optical system that uses a visual field conversion optical element, the optical system described in Japanese Laid Open Patent Application No. H2-69710 is known. This prior art optical system combines a negative lens element, a visual field conversion optical element, a positive lens element and a positive lens element. However, since the refractive power of the negative lens element is weak, the correction of field curvature is insufficient. Moreover, since the Abbe number of a positive lens element is small, the amount of chromatic aberration is large.
Another prior art example of an objective optical system that uses a visual field conversion optical element is disclosed in Japanese Laid Open Patent Application No. H5-307139. This optical system consists of a negative lens element, a visual field conversion optical element, a positive lens element, and a positive lens element. However, the refractive index of the negative lens element is small and the generation of lateral color is large.
Another prior art example of a visual field conversion optical element is disclosed in Japanese Laid Open Patent Application No. 37-9530.